Explain how the pupil reflex protects the eye when a person moves from a dark room into bright light. Include the structures involved and the type of response produced.

AQA A-Level Biology (7402) — 3.6.1 Stimuli and Responses · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Bright light is the stimulus detected by receptors in the eye (retina). A nerve impulse is sent along a sensory neurone to the brain (coordinator). A motor neurone carries the impulse to the iris (effector – the circular muscles of the iris). The circular muscles of the iris contract, causing the pupil to constrict and become smaller. This is an automatic, involuntary response that protects the eye by reducing the amount of light entering and preventing damage to the retina.

Examiner tips

  • Use the word "stimulus" for bright light and "response" for pupil constriction.
  • Mention the retina, sensory neurone, brain, motor neurone, iris and circular muscles.
  • Show the sequence: stimulus → sensory neurone → brain → motor neurone → iris.
  • State that the response is involuntary and protects the retina.

Common mistakes

  • Confusing the iris with the cornea or saying the pupil dilates instead of constricts.
  • Omitting the role of the brain as the coordinator or the circular muscles of the iris as the effector.
  • Using vague terms like "eye" instead of specific structures such as retina or iris.

Mark scheme (5 marks)

  1. Bright light is the stimulus detected by receptors in the eye (retina)
  2. A nerve impulse is sent along a sensory neurone to the brain (coordinator)
  3. Motor neurone carries impulse to the iris (effector / circular muscles of the iris)
  4. Circular muscles of the iris contract, causing the pupil to constrict / become smaller
  5. This is an automatic / involuntary response that protects the eye by reducing the amount of light entering / preventing damage to the retina

Key terms in this question

reflex · pupil

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